Electronic devices have been digitized recently, and this market trend demands capacitors used in those devices to have a low impedance at a high frequency region and a greater capacity in a downsized body. In order to meet this demand, plastic-film capacitors, mica-capacitors, or laminated ceramic capacitors are used. Other than those capacitors, aluminum electrolytic capacitors, aluminum solid electrolytic capacitors, tantalum solid electrolytic capacitors are also used for meeting the foregoing demand.
An aluminum solid electrolytic capacitor is formed by this method: positive and negative electrodes etched and made of aluminum foil are wound up with a separator therebetween, and a liquid electrolyte is used. An aluminum solid electrolytic capacitor and a tantalum solid electrolytic capacitor aim to improve the capacitor properties at a high-frequency region. The electrolyte of those capacitors is made of solid electrolyte such as conductive polymer or manganese oxide; the conductive polymer is formed by polymerizing polymeric monomer such as pyrrole or thiophene derivatives. Those solid electrolytic capacitors have been developed and are now available in the market.
FIGS. 6A and 6B show structure of a capacitor element to be used in solid electrolytic capacitors. FIG. 6A shows a perspective view of the capacitor element and FIG. 6B shows a sectional view of the element shown in FIG. 6A taken along line 6B—6B. Valve metal 31 is roughened by etching process, and has anodic oxide film 32 (hereinafter referred to simply as “film”) on its surface. Insulating tape 33 disposed on film 32 divides valve metal 31 into anode leader 31A and capacitor element section 31B. On the surface of film 32 of capacitor element section 31B, the following two layers are formed in this order: solid electrolyte layer 34 made of conductive polymer, and conductive layer 35 made of a carbon layer and a silver paste layer. Capacitor element 36 is thus constructed.
Anode leader 31A and conductive layer 35 are coupled to an anode terminal and a cathode terminal respectively (not shown). The whole capacitor element 36 is covered by an outer casing resin (not shown) formed by molding, so that a solid electrolytic capacitor is obtained.
An electrolytic oxidation polymerization method and a chemical oxidation polymerization method are known as methods of forming solid electrolyte layer 34. According to the former method, a manganese dioxide layer is formed in advance on film 32, and solid electrolyte layer 34 is formed on the manganese dioxide layer. According to the latter method, solid electrolyte layer 34 is formed on film 32 directly.
The carbon layer and the silver paste layer are formed by applying the respective pastes available in the market and drying them.
The prior art related to the present invention is Japanese Patent Application Unexamined Publication No. H05-159987.
The characteristics of the foregoing solid electrolytic capacitor largely depend on conductive layer 35 made of the carbon layer and the silver paste layer formed on the surface of solid electrolyte layer 34. In particular, material of silver particles of the silver paste layer, its particle shape, a ratio of resin material vs. silver particles affect an equivalent series resistance (hereinafter referred to as “ESR”) of the capacitor characteristics.
However, optimization of the silver particles material and its particle shape as well as the ratio of the silver particles vs. epoxy resin (reactant of bisphenol A and epichlorohydrin) available in the market cannot achieve a capacitor that satisfies the characteristics in a high frequency region needed for the digitization of electronic devices.
An interface resistance between the carbon layer and the silver paste layer becomes high depending on a surface condition of solid electrolyte layer 34, so that the ESR of the capacitor becomes high.